Contrasting evaluation of anaerobic digestion of paper sludge: the role of initial pH and temperature in biogas production and COD removal

Abstract Anaerobic digestion represents a promising technology for the recovery of biodegradable waste, including paper sludge. This study experimentally evaluates the influence of temperature and pH on biogas production and the removal efficiency of organic matter, expressed as COD elimination, during the anaerobic digestion of paper sludge from recycled paper. Using a microbial consortium derived from an operational paper mill digester, the experiments simulated realistic process conditions. Previous studies examined the possibilities of using paper sludge. In our research, we focused on the contrasting optimisation of anaerobic digestion of paper sludge (when changing pH and temperature) for energy versus COD removal. The results demonstrated a strong influence of both parameters on the course and efficiency of digestion. The highest cumulative biogas yield (115 m 3 ·t −1 VS) was obtained at 35 °C and pH 7.5, representing a more than threefold increase compared to 20 °C (28 m 3 ·t −1 VS). In contrast, the highest COD removal (75.23%) was achieved at 20 °C and pH 6.2, indicating that optimal conditions for maximum energy yield and pollutant removal differ. Low pH values (6.2–6.8), particularly at higher temperature, caused methanogenic inhibition, highlighting the microbial community’s sensitivity to acidic environments. The findings contribute to a better understanding of paper sludge behaviour as a substrate in anaerobic digestion and confirm its potential as a renewable energy source. Modelling results emphasised that maintaining mesophilic temperature and optimal pH control is essential to maximise biogas yield, while extending digestion time under balanced conditions promotes efficient COD reduction.